In driving an acoustic speaker, a linear power amplifying apparatus such as an A-class, B-class, or AB-class amplifying apparatus having a simple configuration is used popularly. In such a linear power amplifying apparatus, a power loss of the power amplifying apparatus itself is large in an operational principle, and heat generated by a power consumption of the power amplifying apparatus itself increases according to an increase in output power. For this reason, a large heat sink to diffuse the heat is disadvantageously necessary. Therefore, as a power amplifying apparatus having a large output power, a switching power amplifying apparatus called a D-class power amplifying apparatus has been used.
A power amplifying apparatus having a D-class amplification function turns on or off an output power switch which supplies a power to switch a positive power supply voltage, a zero voltage, or a negative power supply voltage and to generate the switched voltage between output terminals. An inaudible high-frequency band power is removed by a power low-pass filter (LPF) arranged between the output terminals and a load to supply only an audible band power to the load. In a switch-on state, although a current flows, and an inter-terminal voltage is very small. In an off state, although a voltage is applied, a passing current is almost zero. For this reason, all power consumptions, which are products of the voltages and the currents, of the switches themselves are small.
In such a power amplifying apparatus, an output AC (alternate current) signal Vo varies due to a variation in power supply voltage. As a method of solving the problem, for example, there is a technique (for example, see Patent Document 1) that proportionates an amplitude Et of a triangular wave voltage Vt for pulse width modulation of a drive pulse for turning on or off a power switch to a supply voltage Vc. In addition, as conventional arts, there are patent documents 2, 3 and 4.
As a technique related to a power amplifying apparatus having a D-class amplification function, as shown in FIG. 6A, a power amplifying apparatus for driving loads in four switch circuits having an H-shaped bridge configuration, so-called a bridge-tied load (to be referred to as a “BTL” hereinafter) is generally known. The configuration and the operation of the power amplifying apparatus shown in FIG. 6A will be briefly described below.
A first switch circuit 11 to which a DC power supply 10 supplies a voltage Vc is composed of a first high-side switch 111 and a first low-side switch 112 which are n-channel MOSFETs. Similarly, a second switch circuit 12 is composed of a second high-side switch 121 and a second low-side switch 122 which are N-channel MOSFETs. An output terminal of the first switch circuit 11, i.e., a connection point between the first high-side switch 111 and the first low-side switch 112 is defined as a terminal X, and an output terminal of the second switch circuit 12, i.e., a connection point between the second high-side switch 121 and the second low-side switch 122 is defined as a terminal Y. A series circuit of an inductor 13 and a load 14 is connected between the terminal X and the terminal Y.
A control circuit 150 controls the first switch circuit 11 and the second switch circuit 12, and includes a pulse width modulation (PWM) circuit 40, a first drive circuit 51, and a second drive circuit 52. A signal source 16 outputs an input AC signal Vi.
The PWM circuit 40 converts the amplitude of an input AC signal Vi into a pulse width. An comparator 41 compares a triangular wave voltage Vt generated by a triangular wave generation circuit 300 with the input AC signal Vi to output the result as a signal M1. An inverter 42A inverts the signal M1 to output a signal M2.
The first drive circuit 51 includes an amplifier 511 which receives the signal M1 to drive the first high-side switch 111 and an inversion amplifier 512 which receives the signal M1 to drive the first low-side switch 112. The second drive circuit 52 includes an amplifier 521 which receives the signal M2 to drive the second high-side switch 121 and an inversion amplifier 522 which receives the signal M2 to drive the second low-side switch 122.
FIG. 6B is a timing chart of a conventional power amplifying apparatus constituted as described above.
As shown in FIG. 6B, a triangular wave voltage Vt increases and decreases between voltages ±Et having an amplitude Et in a cycle T. The cycle T is set to be sufficiently shorter than the change of the input DC signal Vi. The triangular wave voltage Vt and the input AC signal Vi are compared with each other by the comparator 41. An inverter 42 inverts the output signal M1 from the comparator 41 to generate a signal M2. The signal M1 goes to a high (H) level when the instantaneous value of the triangular wave voltage Vt is smaller than the instantaneous value of the input AC signal Vi, i.e., Vt(t)<Vi(t). A ratio δ (duty ratio) of the period in which the signal M1 is set at the H level to the cycle T is expressed by the following equation.δ=(1+Vi/Et)/2  (1)
The first high-side switch 111 is turned on or off depending on the signal M1, and the first low-side switch 112 is turned on or off depending on the inverted signal of the signal M1. More specifically, the first high-side switch 111 and the first low-side switch 112 in the first switch circuit 11 are alternately turned on or off. On the other hand, the second high-side switch 121 is turned on or off depending on the signal M2, and the second low-side switch 122 is turned on or off depending on the inverted signal of the signal M2. More specifically, the second high-side switch 121 and the second low-side switch 122 in the second switch circuit 12 perform on/off operations opposite to those in the first switch circuit 11, respectively.
Therefore, in a period in which the signal M1 is set at H level, the terminal X has a voltage Vc which is a voltage at one end of the DC power supply 10, and the terminal Y has a voltage 0, i.e. zero potential, which is a voltage at the other terminal of the DC power supply 10. In a period in which the signal M1 is at a low (L) level, the terminal X has zero potential, and the terminal Y has the power supply voltage Vc. The above switching operation is repeated in the cycle T of the triangular wave voltage Vt. The cycle T is set to be so short that a variation of the input AC signal Vi can be neglected. Thus, an average potential Vx of a pulse voltage generated at the terminal X and an average potential Vy at the other terminal Y are expressed by using the duty ratio δ of the signal M1, the following equations can be obtained:Vx=δVcVy=(1δ)Vc.
A smoothing function achieved by the inductor 13 generates a differential voltage between the average voltage Vx and the average voltage Vy across the terminals of the load 14. A voltage across the terminals, i.e., the output AC signal Vo is expressed by the following equation.Vo=Vx−Vy=(2δ−1)Vc  (2)
Equation (1) is assigned to equation (2) to obtainVo=(Vc/Et)δVi  (3).More specifically, the output AC signal Vo is equal to a voltage obtained by amplifying the input AC signal Vi, (Vc/Et) times.
In this manner, in the conventional technique shown in FIGS. 6A and 6B, the input AC signal Vi is modulated in pulse width by the PWM circuit 40 and amplified by an output section of the BTL, resulting in an overall gain of (Vc/Et). The value Et in equation (3) is a modulation sensitivity of a pulse modulation unit related to a part of the PWM circuit 40, and the value Vc is a gain element of the BTL output section.
A ripple variation caused by an output internal resistance when a large current is supplied to the load 14 or a ripple component remaining when a commercial power supply voltage is rectified is superposed on the voltage Vc of the DC power supply 10. In order to reduce the ripple variation or the ripple component, a circuit scale or a loss power has to be increased. In the configuration of the power amplifying apparatus in FIG. 6A, an amplification factor (Vc/Et) varies according to variation in Vc to increase level variation or distortion of the output AC signal Vo.
For improvement of the variation of the output AC signal Vo caused by the variation of the power supply voltage, for example, as disclosed in Patent Document 1, a technique that proportionates the amplitude Et of the triangular wave voltage Vt to the power supply voltage Vc is known. FIG. 7A shows a circuit diagram of a triangular wave generation circuit described in Patent Document 1 and an operation waveform chart of the circuit diagrams.
The configuration and the operation of the triangular wave generation circuit in FIG. 7A will be described below. In FIG. 7A, a terminal A1 is a terminal which receives a DC power supply voltage Vc, and is connected to a resistor R1. Alphabetical symbol “ADD” denotes an operational amplifier to which a resistor R2 and a resistor R3 are connected for providing an operation of an inverting amplifier. Alphabetical symbols “Cx” and “Cy” denote comparators. Alphabetical symbol “FF” denotes a flipflop. Alphabetical symbol “INT” denotes an operational amplifier to which a resistor R0 and a capacitor C0 are connected for providing an operation of an analog integrator. An output from the analog integrator INT is the triangular wave voltage Vt.
The gain of the operational amplifier ADD is sufficiently large and negative feedback is performed by the resistor R3, and thus the operational amplifier ADD operates such that a potential difference is rarely generated between the positive and negative input terminals. Hence the potential at a connection point G between the resistor R2 and the resistor R3 is a zero potential. Therefore, a potential Va2 at a connection point A2 between the resistor R1 and the resistor R2 is equal to a potential obtained by dividing the DC power supply voltage Vc by the resistor R1 and the resistor R2, and is expressed by the following equation.Va2=Vc·R2/(R1+R2)  (4)
If resistances of the resistor R2 and the resistor R3 are equal to each other, a potential Va3 at an output terminal A3 of the operational amplifier ADD is a potential obtained by inverting the potential at the connection point A2 as expressed by the following equation.Va3=−Va2=−Vc·R2/(R1+R2)  (5)
On the other hand, in the operational amplifier INT, the flipflop FF is set. As shown by the broken line in FIG. 7B, when an output Q (voltage at a connection point A4) is a positive predetermined voltage (Vf), the voltage Vf is integrated. As a result, an output Vt linearly decreases. On the other hand, when the flipflop FF is reset and the connection point A4 is a negative predetermined voltage (−Vf), the output Vt linearly increases. It is noted that in this case, when the output Vt is equal to the voltage (Va2) at the connection point A2, the flipflop FF is set by the comparator Cx. When the output Vt is equal to the voltage (−Va2) at a connection point A3, the flipflop FF is reset by the comparator Cy. Therefore, the output Vt is a triangular voltage which varies between voltages ±Va2. As described in equations (4) and (5), the amplitude of the triangular wave voltage is proportional to the power supply voltage Vc.
The voltage Et of equation (3) is equal to the voltage Va2 expressed by equations (4) and (5). Thus, the voltages Va2 of the equation (4) and equation (5) is assigned to the voltage Et of equation (3) to obtain the following equation.Vo=(Vc/Va2)·Vi=(1+R1/R2)·Vi  (6)
In this manner, when the amplitude of the triangular wave voltage Vt is made proportional to the power supply voltage Vc, the amplification factor of the D-class power amplifying apparatus can be made constant without being affected by the power supply voltage Vc.
<Patent Documents>
    Patent Document 1: JP,54-80657,A (see FIG. 4)    Patent Document 2: JP,60-190010,A    Patent Document 3: JP,2002-64983,A    Patent Document 4: JP,61-39708,A    Patent Document 5: JP,3-159409,A